Across the world there are significant deposits and quantities of sulfidic materials including economically desirable metals to recover, especially precious metals such as gold and silver. For example, there are significant deposits and stockpiles of pyritic ores including gold and/or silver and other precious metals such as platinum and platinum group metals.
Some of these deposits are contaminated with difficult to process contaminants such as arsenic, antimony, bismuth or other heavy metals. Ore treatment may also be complicated when high levels of carbon are present, as carbon associates with and has a high affinity for precious metals such as gold.
Current commercially available options for the oxidation of sulfidic materials include roasting, pressure oxidation (POx) and bio-oxidation (Biox). In the POx and Biox processes a sulfate medium is typically employed.
Roasting sulfidic ores presents significant problems due to emissions of environmentally toxic sulfur based gases (so-called SOx gases). Where arsenic is present in the ore, poisonous substances such as arsenic trioxide are produced. For these reasons international trends are to move away from roasting of sulfide ores.
Pressure oxidation of sulfidic materials is employed to avoid the problems of roasting, but requires high pressures (typically greater than 30 bar) and relatively high temperatures (greater than 200° C.). Pressure oxidation is also typically carried out in a sulfate based solution.
U.S. Pat. No. 6,461,577 discloses a bio-oxidation process for treating sulfidic materials containing arsenic where the sulfidic material is subjected to a two-stage Biox process to solubilise the arsenic. The configuration of the leaching process is complex, as is the use of bio-leaching bacteria In addition, bio-oxidation is notoriously slow.
U.S. Pat. No. 4,053,305 discloses a leaching process for the recovery of copper and silver from a sulfide ore using a combination of ferrous chloride solution and pressurised oxygen. Whilst copper is dissolved in the leach, silver is deliberately not leached and is passed with the solid residue from the leach. The silver must then be extracted from the residue using sodium cyanide, an environmentally harmful leaching agent.
U.S. Pat. No. 4,410,496 discloses a leaching process for the recovery of copper, lead and zinc from a sulfide ore using a combination of calcium or barium chloride solution and pressurised oxygen. Again, precious metals in the ore remain unleached and pass with the solid residue from the leach and must be separately extracted.
U.S. Pat. No. 4,655,829 discloses a leaching process for the recovery of metals from a sulfide ore that comprises arsenic and antimony. In this process a bulk sulfide concentrate is prepared from the arsenic sulfide ore. The concentrate is slurried in excess calcium chloride solution. Once the concentrate is prepared, the total metal content and composition of the concentrate needs to be determined. To prevent soluble arsenic compounds or toxic arsenic vapours being created in the process the concentrate is blended with a balancing solution slurry containing a predetermined concentration of copper, lead, zinc, or a mixture thereof in the form of sulfides of those metals. In this regard, the concentrate and the balancing solution slurry are combined to form a reaction slurry having a predetermined metal content such that the molar concentration of arsenic and antimony in the mixture is about equal to the molar concentration of copper, lead, and zinc, ranging from between about 60-40 or 40-60. Only once the mixture is properly balanced is it heated and aerated under pressure to oxidise the metals to soluble components. In other words, the balancing is essential so that no soluble arsenic compounds or toxic arsenic vapours are created.
It would be advantageous if a simple hydrometallurgical process could be provided for recovering a precious metal, especially gold, from a sulfidic material.